This invention relates to a solid phosphoric acid (SPA) catalyst comprising silicon orthophosphate and optionally silicon pyrophosphate. The catalyst is characterized by a pore volume of at least about 0.17 cm3 g−1, of which at least about 0.15 cm3 g−1 is due to macropores with diameters greater than about 10,000 Å. The integrated XRD reflectance intensity ratio of silicon orthophosphate to silicon pyrophosphate, if the latter is present, is at least about 5:1 and preferably at least about 8:1.
Solid phosphoric acid catalysts are commonly used in hydrocarbon conversion processes that require a strongly acidic catalyst. Examples of hydrocarbon conversion processes in which solid phosphoric acid catalysts have been used include, without limitation, the oligomerization of light olefins to a mixture of heavier olefins and paraffins (“polymer gasoline” or “polygas”) and the alkylation of benzene and other aromatic hydrocarbons with olefins to produce alkyl aromatic products such as cumene and ethylbenzene.
The basic recipe for solid phosphoric acid catalysts is disclosed in U.S. Pat. No. 1,993,513, which discloses a catalyst prepared from 89% aq. phosphoric acid and kieselguhr. A mixture of these two ingredients is calcined and ground down to the desired particle size; the “structure” of the catalyst may be improved by adding an organic binder such as starch or gelatin before calcining. Various improvements on this process have been developed over the years. For example, U.S. Pat. No. 3,112,350 teaches a process in which phosphoric acid of 84.8% P2O5 content is added to diatomaceous earth in an approximately 4:1 ratio. The resulting mixture is extruded, cut into manageable pieces, dried, and calcined.
In very general terms, SPA catalysts comprise a phosphorus source and a silicon source. The phosphorus source, which typically contributes from about 60% to about 80% of the catalyst by weight, is generally some sort of “phosphoric acid.” The phosphoric acids are oxyacids of phosphorus in the +5 oxidation state, and have the generic formula Hn+2PnO3n+1. The first three acids in this series are: orthophosphoric acid H3PO4, pyrophosphoric acid H4P2O7, and triphosphoric acid H5P3O10. A given sample of “phosphoric acid” will be a mixture of members of the Hn+2PnO3n+1 series and water. The mixture is characterized by the total phosphorus content, which is given as a percentage relative to pure orthophosphoric acid, H3PO4. As the other acids in the series Hn+2PnO3n+1 have a higher phosphorus content (by weight) than orthophosphoric acid, it is not unusual to find phosphoric acids with concentration greater than 100%. Typically, a phosphoric acid of concentration between about 100% and about 120% is used to prepare SPA catalysts.
The silicon source is a siliceous or SiO2-containing material such as kieselguhr, diatomaceous earth, infusorial earth, kaolin, fullers earth, artificially prepared porous silica, or mixtures thereof. Kieselguhr is the most preferred silicon source. However, the terms infusorial earth, kieselguhr, and diatomaceous earth, are often used and referred to interchangeably and on an equivalent basis in general in reference to SPA catalysts.
It is known in the art that the ratio of crystalline phases, i.e. silicon orthophosphate Si3(PO4)4 and silicon pyrophosphate Si2P2O7, in the finished catalyst affects performance. It is also known that the effectiveness of a catalyst is related to the porosity of solid phosphoric acid catalysts. The ratio of crystalline phases may be controlled indirectly by adjusting conditions in the catalyst preparation process, such as the ratio of phosphoric acid to kieselguhr and the calcination temperature. Similarly, the conditions used for the catalyst preparation can affect the finished catalyst porosity.
The term “porosity” as applied to SPA catalysts encompasses both the total pore volume and the distribution of pores of various sizes. The pore size distribution is often described relative to pore volume. That is, a certain percentage of the pore volume is due to, or is contributed by, pores in a certain diameter range: for example, one might say that 80% of the total pore volume is due to pores with diameters >1000 Å. U.S. Pat. No. 3,661,801 teaches a spherical catalyst (not an extrudate) prepared using non-hydrated P2O5 and having between about 0.200 and about 0.400 cm3 g−1 of pore volume contributed by pores with diameter >350 Å and between about 0.07 and about 0.20 cm3 g−1 of pore volume contributed by pores with diameter >9000 Å. U.S. Pat. No. 5,081,086 teaches a solid phosphoric acid catalyst with a total pore volume of 0.28 cm3 g−1 or less, with no more than 25% of the pore volume contributed by pores with diameter >10,000 Å. It is stated in the '086 patent that pores with diameters above 10,000 Å should not contribute a large percentage of the total pore volume, as these large pores are detrimental to the physical strength and longevity of the catalyst. European Patent EP 570,070 B1 teaches a solid phosphoric acid catalyst having an integrated XRD reflectance intensity ratio of silicon orthophosphate to silicon pyrophosphate which is less than about 4:1, and having at least 30% of its total pore volume contributed by pores with diameter >10,000 Å.
In order for the SPA catalysts to function in hydrocarbon conversion processes, the catalysts must have efficient mass transfer and resistance to deactivation, as well as high activity. Toward this end, it would be beneficial to have a SPA catalyst that has an effective ratio of silicon orthophosphate to silicon pyrophosphate, and a pore structure having a sufficient volume of large macropores, with diameters above about 10,000 Å, and preferably above about 50,000 Å.